The field of the invention is control systems for dual inertia lost motion mechanical systems and more specifically optimal acceleration feedback for use by a controller in controlling a system characterized by lost motion.
This section of this document is intended to introduce various aspects of art that may be related to the present invention described and/or claimed below and provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
High performance servo drives are used in many different applications and in many different industries. For example, in the media printing industry servo drives are routinely employed to accelerate and decelerate large spools of paper web to wind and unwind and position sections of the web precisely with respect to printer heads and other system components to facilitate application of information on the paper material surfaces. In printing applications and in many other applications precise motor-load control is extremely important.
Typically a velocity command signal is provided to a drive indicating a desired motor load velocity and the drive is configured to apply AC voltages to the motor causing motor velocity to converge toward the command velocity. The relationship between applied voltages and motor velocity is effected by motor-load assembly inertia. For instance, assume first and second motor-load assemblies where a first assembly inertia is half as large as a second assembly inertia. When motor velocity is to be reduced, a much smaller force is required to increase the first assembly than to slow the second assembly at the same rate. Similarly, when motor velocity is to be increased, a much larger force is required to increase the first assembly velocity than to increase the second assembly velocity at the same rate.
Many systems include one or more feedback loops that provide motor-load operating characteristics to the drive for comparison to the command velocity and derivatives thereof so that the drive can adjust motor velocity in a suitable fashion and increase performance. Thus, for instance, some drives include a position feedback loop and/or a velocity feedback loop. In these cases, when the motor position or velocity do not match the commanded position (e.g., the integral of the commanded velocity) and/or the commanded velocity, the drive alters the applied voltages to correct the error.
In theory, in an ideal directly linked motor-load assembly, the load is linked to the motor via an infinitely stiff coupling and the motor and load inertias appear as a single mass or total assembly inertia. A control system for a motor-load assembly including an infinitely stiff coupling is easily constructed with the application of velocity and/or position feedback loops.
In reality, most direct linked motor-load assemblies do not include an infinitely stiff coupling and instead are characterized by some degree of flexibility. In these cases the flexible motor-load coupling can be thought of as a spring between the motor and load masses such that angular load position typically is slightly different than the angular position of the motor.
When a drive includes one or more feedback loops and a motor-load assembly includes a flexible coupling, the combination often causes resonance within the drive-motor-load system. To this end, when a velocity command is received and a drive accelerates a motor toward a command velocity, in the case of a flexible coupling, the motor achieves the desired velocity prior to the lagging load. Because of the spring effect of the flexible coupling, the load often shoots past the command velocity thereby loading the coupling with another spring force that places a torque on the motor which causes the motor velocity to exceed the commanded velocity so that a velocity error is identified by the control system in the drive (e.g., the feedback velocity instantaneously exceeds the commanded velocity). The drive in turn alters the applied voltages to slow the motor-load assembly.
Eventually the spring force within the coupling causes the load velocity to slow such that the load velocity converges toward the motor velocity. However, here again, as the force within the coupling is released, the load velocity may shoot through the motor velocity thereby placing a slowing force on the motor. In response to the slowing motor velocity the drive again alters the applied voltages, this time to increase the motor speed. This oscillating process continues at what is typically referred to as a system resonant frequency.
While many motor-load assemblies include a direct linkage between the motor and the load, other assemblies include some type of indirect coupling such as a gear box, gear train, etc., that introduces lost motion between the motor and the load. Here, the phrase “lost motion” is used to refer to an actual disconnect between the motor and the load that occurs under certain operating circumstances because there is some mechanical “play” or “slack” within the coupling components. For instance, in a simple example, a particularly sloppy gear train may result in a coupling where five degrees of motor rotation separate the relative positions in which the motor engages the load through the gear train in opposite directions. Thus, when rotating in a forward direction the motor-gear train may be engaged with the load at a first angle and the motor may have to be rotated five degrees in the reverse direction relative to the load to engage the load in the other direction. In this case, assume that the motor is initially rotating in the forward direction to drive a coupled load and that the command velocity is reduced to slow motor speed. Unless load friction slows the load at a rate at least as fast as the rate at which the motor speed is reduced, at the instant that the motor speed is reduced, the motor and load become decoupled.
Motor-load assemblies that include lost motion couplings add complex dynamics to drive systems that complicate control tasks appreciably. In this regard, the instantaneous effective inertia associated with the motor changes whenever a lost motion coupling causes the motor-load to couple or decouple. Here, the phrase “effective inertia” is used to refer to the motor inertia alone when the load is decoupled from the motor and to the sum of the motor inertia and the load inertia when the load is coupled to the motor. In many cases the load inertia will be much larger than the motor inertia so that the difference between the effective coupled inertia and effective decoupled inertia is appreciable.
Because inertia effects the relationship between applied voltages and velocity and the effective inertias are different during coupled and decoupled motor-load conditions, drive velocity adjustments that occur during coupled and decoupled motor-load conditions end up causing different velocity modifications. Thus, for instance, given a specific velocity error, independent of whether or not the error occurs when the motor is coupled to the load and when the motor is decoupled from the load, the error will result in the same change in applied voltages to eliminate the error. Here, however, because the coupled and decoupled inertias are different, the effect on velocity is different. Thus each of spring related resonance and lost motion coupling dynamics hamper motor-load assembly control.
One way to eliminate system resonance is to provide filters within the feedback loops that filter out disturbances at the resonant frequencies. Thus, for instance, where a system resonant frequency is known, a notch filter may be provided that specifically eliminates the resonant frequency signal from the feedback loop to the drive.
While filters are routinely used to reduce system resonance, such filters typically have not been employed to minimize the effects of lost motion couplings because they generally do not provide the bandwidth necessary to meet application requirements.